1. Field of the Invention
The present invention relates to a power transmitting system of a vehicle, wherein a differential mechanism, a power transmitting mechanism and a dog clutch are disposed between a drive power source and drive wheels.
2. Description of Related Art
There is well known a power transmitting system of a vehicle, comprising a differential mechanism, a clutch mechanism, a power transmitting mechanism and a dog clutch which are disposed between an input rotary member provided to receive a drive force from a drive power source of the vehicle and an output rotary member provided to transmit the drive force to drive wheels of the vehicle. The differential mechanism includes an input rotary element, an output rotary element and a reaction rotary element, and the clutch mechanism selectively connects two rotary elements of the input, output and reaction rotary elements of the differential mechanism, to each other. The power transmitting mechanism has a predetermined gear ratio, and the dog clutch is configured to selectively place a power transmitting path between the differential mechanism and the output rotary member, in a power transmitting state and a power cutoff state. WO/2013/176208 A discloses an example of such a vehicle power transmitting system, which has a power transmitting path provided with a continuously variable transmission mechanism, and a power transmitting path provided with a gear mechanism. These two power transmitting paths are arranged in parallel with each other between an input shaft and an output shaft of the power transmitting system. In the power transmitting path provided with the gear mechanism, a forward/reverse switching mechanism, a gear train and a dog clutch (claw clutch or positive clutch) are disposed in this order of description between the input and output shafts, in a direction from the input shaft toward the output shaft. The forward/reverse switching mechanism is provided with a planetary gear set, and a forward drive clutch for selectively connecting two rotary elements of the planetary gear set to each other. This vehicle power transmitting system can be configured such that a speed ratio of the power transmitting path provided with the gear mechanism is higher than a highest speed ratio (corresponding to the lowest gear position) of the power transmitting path provided with the continuously variable transmission mechanism. This configuration permits the vehicle to be driven with a large drive force upon starting of the vehicle, for instance, when the forward drive clutch and the dog clutch are both placed in engaged states to select the power transmitting path provided with the gear mechanism.
By the way, it is considered to control the dog clutch such that the dog clutch is held in its released state while an engine is held at rest, in view of a possibility that the vehicle is towed, and is switched to its engaged state when the engine is started. According to this control of the dog clutch, there is a risk of a failure to bring the dog clutch into the engaged state (namely, an “up-lock” of the dog clutch) due to a failure of meshing of spline teeth of the dog clutch (synchro-mesh mechanism) in abutting contact of tooth faces with each other, which may take place upon a so-called “garage shifting” action (from a parking position P to a drive position D, for example) of a shift lever just after the engine is started. This phenomenon results in a failure to speedily start the vehicle. Where the power transmitting path provided with the gear mechanism has a high speed ratio and the dog clutch is controlled to be held in the engaged state while the engine is held at rest, so that the dog clutch is ready for starting the vehicle, on the other hand, a rotary motion the speed of which is raised when the vehicle is towed is input from the output rotary member to the differential mechanism, so that the differential mechanism has a large difference among rotating speeds of its rotary elements, giving rise to a risk of deterioration of durability of the differential mechanism due to excessively high rotating speeds of the rotary elements such as a pinion gear. In this respect, it is noted that the problems described above had not been publicly recognized at the time the present invention was made.